The term mystery pitcher was coined by Philadelphia Inquirer baseball columnist Jayson Stark. It refers to a position player who comes in to pitch in the late innings of a blowout loss. Sometimes, a mystery pitcher is forced to enter an extra inning game when the bullpen has been emptied of available pitchers. The most recent mystery pitchers to win a game were Brent Mayne in 2000 and Wilson Valdez in 2011. Stevie Wilkerson also made history as a mystery pitcher when he became the first to record a save in 2019. They are also sometimes known as "joke pitchers".
The use of mystery pitchers was very rare until the 1980s, when certain managers began to use the strategy more often as a way to spare their bullpen. This was just an uptick, however, but in the 2010s, the strategy really took off, as there was no longer any stigma attached to it (in earlier times, managers were often accused of making a mockery of the game by their peers or by sportswriters when they resorted to the strategy). For example, from 1969 to 1978, the most instances of the use of the strategy in any season was 3; from 1986 to 1991, there were at least 8 such instances every year, but there was only one season between 1992 and 2008 in which there were as many as 10 (that was 2004, with exactly 10). However, from 2012 onward, the usage blew up, going from 11 in 2008 to 24 in 2015 to an unprecedented 57 in 2018, a total which was shattered in 2019.
Major League Baseball took notice of this trend and began to examine ways to curb it. In 2019, one of the rules changes proposed was that such pitchers could only be used in a game with a margin of 6 or more runs, or in extra innings. The rule was slated to come into effect in 2020; however, it was unlikely to change the burgeoning use of position players on the mound, given that these were precisely the situations in which they were being used already.
- Gabe Lacques: "Garbage time, all the time: MLB sees historic spike in position players pitching", USA Today, August 21, 2019. 
- Sarah Langs: "Best performances by position players pitching", mlb.com, May 12, 2020. 
- Mike Petriello: "Behold the glory of position players pitching", mlb.com, November 15, 2020.